The present invention relates to a hydrogen-evolution electrode which not only exhibits low hydrogen overvoltage and high stability for a long period of time but also is available at low cost, and a method for producing the same. More particularly, the present invention is directed to a hydrogen-evolution electrode which comprises an electrically conductive substrate having thereon a coating layer containing a chromium component and an oxide of at least one metal selected from the group consisting of nickel and cobalt, wherein the content of chromium in the coating layer is 0.5 to 20% in terms of atomic percentage as defined later, and a method for producing the same.
Recently, due to the rapid increase in cost of energy, it has become very important more and more to decrease the superfluous consumption of energy by lowering the hydrogen overvoltage of a hydrogen-evolution electrode used in industrial electrolysis. For this purpose, many researches and developments have been made. For example, in order to decrease the hydrogen overvoltage of a hydrogen-evolution electrode, it has been attempted to increase the surface area of the electrode or to incorporate an electrode catalyst into the electrode, by means of a technique such as an electroplating method, a chemical plating method, a method comprising applying a coating solution followed by baking and melt-spraying. Further, as the technique to increase the surface area of the electrode or to incorporate an electode catalyst into the electrode, there have been proposed a method comprising melt-spraying onto a substrate a powder material containing a sacrificial metal component followed by leaching out of the sacrificial metal component, a method comprising applying a coating solution onto a substrate followed by baking and heating the obtained coating in a reducing atmosphere and the like. There have been proposed, as the electrode catalyst capable of lowering the hydrogen overvoltage, transition metals such as cobalt, molybdenum, vanadium, manganese and tungusten, noble metals such as platinum, silver, ruthenium and iridium or a mixture thereof, or a combination of a metal component selected from the above metals with a sacrificial metal component. The heretofore proposed electrodes include those in which a metal, an alloy or a mixture thereof is used as an active material, and those in which a metal oxide, a compound oxide or a mixture of metal oxides is used as an active material.
As examples of the electrode in which a metal, an alloy or a mixture thereof is used as an active material, there can be mentioned an electrode which comprises a copper substrate bearing on its surface a coating of an alloy of nickel, vanadium and molybdenum formed by plating (U.S. Pat. No. 4,033,837); an electrode having a coating of an alloy of cobalt, molybdenum and vanadium formed by electroplating (Japanese Patent Application Laid-Open Specification No. 33490/1981); an electrode coated with a thin nickel layer containing manganese and sulfur (Japanese Patent Application Publication No. 6715/1980); an electrode which comprises an alloy of a metal selected from nickel and cobalt and a metal selected from titanium and magnesium (Japanese Patent Application Publication No. 44955/1981); an electrode which is produced by a method which comprises applying a homogeneous solution containing a metal selected from the first group consisting of iron, cobalt, nickel and manganese and a metal selected from the second group consisting of molybdenum, vanadium and tungsten, followed by baking to obtain a coating of a mixture of oxides, and heating the obtained coating in a reducing atmosphere to cure it (European Patent Application Laid-Open Specification No. 9406); an electrode prepared by a process which comprises applying onto a substrate a mixture of an alloy of iron or a metallic component composed mainly of iron and an alkali-soluble sacrificial metal by melt-spraying, and subsequently leaching out the sacrificial metal by an alkali-treatment (Japanese Patent Application Laid-Open Specification No. 115984/1980); an electrode prepared by a process which comprises applying onto a substrate a Raney type alloy containing a sacrificial metal by melt-spraying to form a coating, followed by leaching out the sacrificial metal in the melt-sprayed coating by means of alkali-treatment and anodic polarization, and subsequently plating its surface with a metal which exhibits low hydrogen overvoltage (Japanese Patent Application Laid-Open Specification No. 122887/1980); an electrode prepared by a process which comprises applying onto a substrate a mixture of nickel and a water-soluble compound capable of being melt-sprayed by melt-spraying to form a coating and leaching out the compound to form a porous nickel coating (Japanese Patent Application Laid-Open Specification No. 188689/1982); an electrode bearing on its surface a coating prepared by melt-spraying a powder material exhibiting low hydrogen overvoltage, said powder material being a powder of metal selected from the group consisting of cobalt, nickel, platinum, molybdenum, tungsten, manganese, iron, tantalum and niobium, a carbide thereof, a nitride thereof, an aluminide thereof and an alloy thereof, and a mixture thereof (U.S. Pat. No. 4,049,841); and an electrode prepared by a process which comprises applying onto a substrate a mixture of particulate nickel, particulate cobalt and particulate aluminum by melt-spraying to form a coating and removing aluminum from the melt-sprayed coating by leaching (U.S. Pat. No. 4,024,044).
The above-mentioned electrodes in which a metal, an alloy or a mixture thereof is used as an active material have a fatal disadvantage that when electrolysis is continuously effected using the above-mentioned type of electrode as a hydrogen-evolution electrode, the hydrogen overvoltage of the electrode increases with the lapse of time, and the activity of the electrode is lost. Moreover, the electrode of the above type has another disadvantage that, in the electrolysis of a sodium chloride solution according to the ion exchange membrane method using the above-mentioned type of electrode, a metal component of the electrode dissolves out of the electrode by corrosion or the electrode comes into a passive state by oxidation due to the reverse current which occurs at the time of stopping of the electrolysis, whereby the life of the electrode becomes short.
As electrodes eliminating the above-mentioned disadvantages of the electrode of which the active material is a metal or the like, there have been proposed electrodes in which a metal oxide, a compound oxide or a mixture of metal oxides is used as an active material. As examples of the electrodes of the above-mentioned kind, there can be mentioned an electrode of which surface consists essentially of a spinel type compound oxide to be used in producing hydrogen by electrolysis, in which said compound oxide is represented by the general formula (M.sub.II)(M.sub.III).sub.2 O.sub.4 wherein (M.sub.II) is selected from the group consisting of iron, zinc, manganese, nickel, cobalt, magnesium, cadmium and copper and (M.sub.III) is selected from the group consisting of iron, chromium, manganese, nickel and cobalt and wherein, necessarily, the proportions of M.sub.II and M.sub.III to M.sub.II +M.sub.III, in terms of atomic percentage, are about 33% and about 67%, respectively (U.S. Pat. No. 4,243,497); and an electrode which comprises a titanium-containing non-stoichiometric compound bonded by a metal selected from nickel, cobalt and iron, wherein said titanium-containing non-stoichiometric compound further comprises a compound represented by the formula A.sub.x Ti.sub.y O.sub.z (in which A is an alkali metal) (U.S. Pat. No. 4,080,278). However, such electrodes in which a compound oxide is used as an active material cannot exhibit sufficiently low hydrogen overvoltage. In European Patent Application Laid-Open Specification No. 23368, there is disclosed an electrode which contains a metal oxide prepared by thermally decomposing a thermally decomposable compound of cobalt, iron, manganese or nickel. Such an electrode cannot exhibit sufficiently low hydrogen overvoltage, or the metal oxide of the above-mentioned electrode is reduced to the state of metal by a hydrogen-evolution reaction in a relatively short period of time. The electrode containing the thus reduced metal oxide, similarly to an electrode of which the active material is a metal or the like, has not only such a disadvantage that the hydrogen overvoltage increases in an electrolysis involving a hydrogen-evolution reaction with the lapse of time but also such a disadvantage that the activity of the electrode is lost due to dissolution-out of a metal component caused by reverse current.
Moreover, there has been proposed an electrode having on its surface a coating of a melt-sprayed admixture consisting essentially of particulate cobalt and particulate zirconia (U.S. Pat. No. 3,992,278). Although this electrode comprises a coating of a combination of a metal with a metal oxide, this electrode also cannot exhibit sufficiently low hydrogen overvoltage and therefore, this electrode is also not suitable for use as a hydrogen-evolution electrode. Moreover, the hydrogen-evolution potential of the above-mentioned electrode during the electrolysis is less noble than the electrodeposition potential of iron ions which are dissolved in an electrolyte in a small amount. For this reason, the iron ions are consecutively electrodeposited onto the electrode, so that the effectiveness of the coating of the above-mentioned kind of electrode is lost in a short period of time.
With respect to an industrially useful hydrogen-evolution electrode, it is essentially required that not only the hydrogen overvoltage be sufficiently low but also the activity of the electrode be maintained for a sufficiently long period of time and that the cost of the electrode be low as compared with the profit derived from the electrode. As mentioned above, however, there has not ever been proposed an electrode which satisfies the above-mentioned requirements, especially the requirement as to the life of the electrode.
With a view to developing a practically useful and economical hydrogen-evolution electrode which not only exhibits low hydrogen overvoltage but also has high durability, the present inventors have made extensive and intensive researches. First, the present inventors' attention was directed to metal oxides which form active sites of a hydrogen-evolution electrode. As examples of the metal oxides forming active sites of the electrode, there may be mentioned oxides of metals such as iron, cobalt, nickel, manganese, chromium, molybdenum, tungusten, vanadium, niobium, tantalum, titanium, zirconium, copper, silver, gold, zinc, cadmium, aluminum, gallium, tin, ruthenium, rhodium, palladium osmium, iridium and platinum and the like. Of them, oxides of nickel and cobalt are preferable from the standpoints of high catalytic activity, stability in an electrolyte and availability as an industrial material. In other words, a hydrogen-evolution electrode having a coating of nickel oxide and/or cobalt oxide is advantageous from the standpoints of high catalytic activity, stability in an electrolyte and availability. However, when such electrode is continuously used in an electrolysis involving a hydrogen-evolution reaction over more than one year, the activity of the electrode tends to be decreased. In order to elucidate the cause of decrease of the activity, the present inventors have made an investigation on the decrease of activity of such electrode. As a result, it has been found that the degree of oxidation of the coating (which will be explained later) is remarkably decreased after a long period of electrolysis. Further, it has been found that when the degree of oxidation of the electrode coating is less than 30%, the hydrogen overvoltage which the electrode exhibits tends to considerably increase with the lapse of time. In order to prevent the decrease of the degree of oxidation of the electrode coating, the present inventors have further made extensive and intensive researches. As a result, it has been found that when at least one member selected from the group consisting of chromium, vanadium, titanium, manganese and niobium and compounds thereof is incorporated into the electrode coating containing an oxide capable of forming active sites, the reduction of the oxide in the coating can be prevented, that is, the decrease of the degree of oxidation of the coating can be prevented. Still further, the present inventors have made an investigation on the influence of the above-mentioned materials on the activity and chemical and electrochemical stability of the electrode. As a result, it has been found that chromium and titanium and compounds thereof are preferable and that chromium and compounds thereof are more preferable. Chromium and compounds thereof are excellent in preventing the decrease of the electrode activity as compared with titanium and compounds thereof.
Based on the above findings, the present inventors have still further made intensive researches with a view to developing a hydrogen-evolution electrode which not only exhibits low hydrogen overvoltage but also has high durability. As a result, it has been found that when the coating of an electrode contains an oxide of at least one metal selected from nickel and cobalt and further contains a chromium component (which will be mentioned later) in a proportion, in terms of atomic percentage, of 0.5 to 20%, the electrode not only exhibits extremely low hydrogen overvoltage but also has high durability.
The present invention has been made based on the above novel findings.
Accordingly, it is one object of the present invention to provide a hydrogen-evolution electrode which not only exhibits low hydrogen overvoltage but also has high durability.
It is another object of the present invention to provide a method of producing a hydrogen-evolution electrode of the kind described above, which can be practiced with ease and high productivity.